Igniting a TNT sulfur cube while a shear/lead is in your offhand makes the offhand swing

When right-clicking a sulfur cube with a TNT block while holding a flint and steel/firecharge in your main hand and a shear/lead in your offhand, the sulfur cube is primed but the offhand does the swing animation. If the item in the offhand is a lead, then the sulfur cube becomes primed and leashed from the same interaction.